1. Field of the Invention
The present invention relates to a frame for a color CRT (Cathode-Ray Tube), in particular to the frame for the color CRT which is capable of increasing an impact character and a vibration character of a shadow mask by strengthening corner portions of the frame.
2. Description of the Prior Art
In general, a frame supporting a shadow mask inside of a color CRT prevents vibrations and distortions of the shadow mask. And, an impact character and a vibration character of the frame can be improved in proportion to stiffness of the frame.
There are many methods for improving the stiffness of the frame, among them increasing thickness of the frame is one of the simple methods.
However, the method is not appropriate because it may cause increase of a unit cost, accordingly product efficiency can be lowered due to that.
Among the methods, a bead forming method is widely used in order to strengthen the stiffness of the frame. In the bead forming method, stiffening beads are formed at a portion imposed stress at the most when vibration displacement or impact displacement occurs.
In the color CRT, particularly corner portions are the portions imposed stress at the most caused by the impact displacement and the vibration displacement.
Hereinafter, the color CRT comprising the conventional frame will now be described with reference to the accompanying drawings.
As depicted in FIG. 1 and FIG. 2, the color CRT comprises a vacuum outer casing including a front panel 1 and a funnel 2 anastomosed on the rear end of the panel 1, a three color fluorescent screen 1a having a red R, green G, blue B color, a shadow mask which is supported by a frame 6 and is installed on the rear portion of the panel so as to have a certain distance from the fluorescent screen 1a in order to make an electron beam 10 throw electron beams accurately on the fluorescent screen.
And, the frame 6 supports inner upper and lower end of the panel 1 by a spring 4, and an inner shield 11 is combined to the rear end of the frame 6 in order to prevent electron beam 10 from being influenced by an outer magnetic field.
The red, green, blue electron beam 10 radiated by a electron gun 3 is deflected toward the fluorescent screen 1a by a vertical horizontal deflection magnetic field of a deflection yoke 8, is subdivided, and can illustrate an image by being thrown on the fluorescent screen 1a. 
Meanwhile, the electron beam 10 radiated by the electron gun 3 has to be accurately thrown the fluorescent screen 1a in accordance with the each color of the screen 1a, when the impact and vibration are transferred from outside to the shadow mask 5, the electron beam 10 can not be thrown accurately on the each color of fluorescent screen 1a, but is deviated, accordingly wave figures occur on the fluorescent screen 1a. 
This is called as a microphonic phenomenon, in order to prevent the phenomenon directly, the stiffness of the shadow mask 5 has to increase, or the phenomenon can be prevented indirectly by strengthening the frame 6.
In addition, a drop impact experiment is performed on the color CRT, when there is distortion of the shadow mask as above, the screen 1a is uneven, accordingly credibility of the product lowers.
As depicted in FIG. 2 and FIG. 3, In order to prevent above mentioned problems, a plurality of beads 6a are protrusively formed at the each inner side and corner of the frame in order to strengthen the stiffness of the frame.
However, as depicted in FIG. 4, most of the stress due to variable vibration modes are concentrated on the corner portions. The each mode shows the vibration distortion in accordance with changes of the frequency.
After all, the beads formed at the each side can not increase the stiffness of the frame.
It is an object of the present invention to provide a frame for a color CRT which is capable of increasing an impact character and a vibration character of a shadow mask by strengthening corner portions of the frame.
The frame for the color CRT of the present invention having a square framework comprises a plurality of beads for strengthening stiffness of the frame formed on the framework including corner portions in order to prevent distortion of the shadow mask.
Herein, the beads on the corner portions are stepped beads having at least two steps.
In addition, the beads on the corner portions are formed along the width direction of the frame.
In addition, the beads on the corner portions are formed along the circumference direction of the frame.
It is advisable for satisfying equation of 0.1xcex11xe2x89xa6xcex12xe2x89xa6[xcex11xe2x88x92(xcex41+xcex42)xe2x88x92R] when the overall length of the stepped bead formed on the long side is xcex11, the length of the lower step of the stepped bead formed on the long side is xcex12, the overall height of the stepped bead formed on the short side is xcex41, the height of the lower step of the stepped bead formed on the short side is xcex42, and radius is R.
It is advisable for ratio of xcex12 about xcex11 to satisfy the range of 0.1 less than xcex12/xcex11 less than 0.8.
It is advisable for ratio of xcex22 about xcex21 to satisfy the range of 0.1 less than (xcex22/xcex21) less than 0.8 when the overall length of the stepped bead formed on the short side is xcex21 and the length of the lower step of the stepped bead formed on the short side is xcex22.
It is advisable for ratio of xcex32 about xcex31 to satisfy the range of 0.2 less than (xcex32/xcex31) less than 0.8. when the overall height of the stepped bead formed on the long side is xcex31 and the height of the lower step of the stepped bead formed on the long side is xcex32.
It is advisable for ratio of h2 about h1 to satisfy the range of 0.2 less than (h2/h1) less than 0.65 when the overall height of the stepped bead formed on the short side is h1 and the height of the lower step of the stepped bead formed on the short side is h2.